The present invention is related generally to primary electrochemical cells and is more particularly concerned with a novel cathode structure for use in such cells which results in high discharge current capability.
Primary electrochemical cells are utilized for a wide variety of applications and are commonly available in a significant assortment of sizes, shapes and discharge capabilities. In some applications, such as electronic watches and heart pacers, a very low discharge current is required. In others, such as portable lighting sources, high discharge current capabilities are often required. This is particularly true with some high intensity portable light sources.
A number of electrochemical systems are known to be incorporable into such primary electrochemical cells and these electrochemical systems afford great variation in discharge capabilities as well. A large portion of these systems utilize a carbon cathode material. Typically, carbon cathode material is formed as a paste and depositied as such within the electrochemical cell container. Because of the fluid form of this paste, albeit viscous, it has been necessary to place a heavy separator between the carbon cathode material and the particular anode material utilized. This separator was required to prevent direct contact between the anode and cathode materials, Necessarily, these separators became quite thick to prevent self-discharging of the cells due to internal shorts between the anode and cathode materials. Such a thick separator naturally increases the internal resistance of the cell thereby decreasing the output current that may be derived therefrom.
An internal construction for primary electrochemical cells which has been utilized for providing such cells with high discharge capabilities is shown in U.S. Pat. No. 3,510,353 to McHenry. In this construction, successive layers of the cathode material, a separator, the anode material, and another separator are rolled, in "jelly roll" fashion so that the anode and cathode materials are in close proximity to each other throughout the volume of the cell. This arrangement is capable of being discharged at relatively high currents. However, heat is released during the electrochemical process. When the anode and cathode materials are dispersed throughout the volume of the cell as in this configuration, the heat is not easily removed from the package and causes a thermal build-up within the cell. In some cell configurations, this heat build-up can become dangerous. For example, in a recently developed class of primary electrochemical cells where a lithium or other alkali metal is utilized as the anode with electrolytic solutions which include a solvent material which can be reduced electrochemically on the surface of the carbon cathode, the thermal build-up can cause the alkali metal to reach its melting temperature at which an extremely rapid reaction between the anode material and the reaction products produced during cell operation can occur. An over-pressurization condition within the cell container is rapidly established and the subsequent rupture thereof is caused.